Formaldehyde can be produced by the catalytic oxidative dehydrogenation of methanol. Processes for carrying out such production are known, for example from WO9632189 or U.S. Pat. No. 2,504,402. The catalyst typically comprises molybdenum and iron oxides. Formaldehyde can also be produced by a mix of catalytic oxidative dehydrogenation and catalytic dehydrogenation of methanol using a silver or copper catalyst. The present invention is concerned with the production of formaldehyde by catalytic oxidative dehydrogenation using mixed oxide catalysts. The reaction to form formaldehyde from methanol over mixed oxide catalysts is exothermic. The reaction may be carried out in an isothermal reactor, typically a tubular reactor, in which the heat of the reaction is removed by a heat transfer fluid. It will be understood that so-called “isothermal” reactors are typically pseudo isothermal in that the temperature varies along the length of the reactor despite the cooling provided to remove the heat of the reaction. Such reactors may also be termed “cooled” reactors. The reaction may also be carried out in an adiabatic reactor, in which heat is not removed and the temperature of the reactor contents increases as they pass through the reactor. Whichever reactor type is used, over time the catalyst ages, becomes less effective and needs replacing. Replacing the catalyst in a tubular reactor may take 4-5 days, during which time the reactor is shut down and revenue is being lost.
Most commercial plants today producing formaldehyde over mixed oxide catalysts are based on isothermal tubular reactors. The isothermal tubular reactor would typically be fed with up to 11 vol % methanol. Above that level a shortage of oxygen can lead to premature aging of the catalyst. The tubular reactors may be arranged in series, which can enable a higher methanol feed to be used than with a single reactor.
An adiabatic ‘post-reactor’ may also be added downstream of the isothermal tubular reactor. Such an adiabatic reactor is aimed at increasing the conversion of the methanol by compensating for inefficiency of the catalyst in the isothermal tubular reactor.
Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art. In particular, preferred embodiments of the present invention seek to provide an improved apparatus and process for the production of formaldehyde.